Taking a closer look into the deep scattering layer - using broadband acoustics to identify mesopelagic organisms in situ

The mesopelagic community has the potential to meet the future food demand by the world’s population. Yet, the biomass and ecological role of the mesopelagic ecosystem are largely unknown. Acoustic techniques are non-invasive and thus ideal to study organisms in situ. Broadband (BB) acoustic systems measure acoustic backscatter from organisms over a wide range of frequencies and increase the range resolution compared to narrowband systems. This increases the ability to separate targets in dense layers and to obtain accurate target strength (TS) measurements, which are essential for deriving acoustic abundance estimates. Furthermore, the increased frequency-response information can give target characterizations such as morphology and size, leading to improved species discrimination.

A submersed towed platform (MESSOR) equipped with BB echo sounders (50 to 260 kHz) was used to measure frequency responses of organisms between 1000 m and the sea surface in the NE Atlantic. Additionally, a macroplankton trawl equipped with a stereo camera system (Deep Vision) provided samples and images of mesopelagic organisms. Acoustic characteristics of various types of organisms are described and we demonstrate that BB acoustics can be used in combination with theoretical scattering models, trawl catches and images to identify and verify different mesopelagic organisms. The acoustic measurements were further used to estimate the abundance of identified organism groups in the deep scattering layer.